Oh totally, luckily my conductor is an awesome guy and he and I have talked about these types of things and he will never put me on the spot for something. My mother though, who is the main pianist gets people asking for her to play stuff right then and there all the time. She's been playing for her whole life though, so she has become very adept at sightreading. Those are the types of people I will tell, if I'm not in a great mood, that if they don't like it then they can go ahead and do it themselves.